What is the output of emerge --info? Does it power off due to running out of battery power? Does it power off due to an orderly shutdown? Does it panic? What changed in the last upgrade? How do you suspend? Is the system resuming itself after the long time period or is it a user-initiated resume?

Is the system resuming itself after the long time period or is it a user-initiated resume?
Resuming itself. I'm not 100% sure when it wakes up, ~8 hours is an estimate for the last time I saw it do that.